This invention relates to an electric vehicle drive train having a unipolar motor.
A unipolar or homopolar motor is a DC dynamoelectric machine that operates at high current and low voltage. It is a motor which has high efficiency and a high power-to-weight to volume ratio. Unipolar motors are described in U.S. Pat. Nos. 3,185,877 to A. Sears; 3,453,467 to L. M. Harvey; 3,916,235 to E. Massar; and 3,984,715 to D. Kullman et al. However, the preferred form of unipolar motor for use in connection with the present invention is described in commonly assigned patent application of the present inventor entitled, "Unipolar Dynamoelectric Machine with Variable Resistance Control" Ser. No. 863100, filed Dec. 1977.
The torque-producing characteristics of the unipolar machine described in the inventor's patent application identified in the preceding paragraph makes this machine particularly suitable for use as a traction motor in electric vehicle applications. However, because unipolar motors operate at low voltage and high current, for example, 10,000 amps, the DC source of electrical energy used to supply such motors must be located in proximity to the motors to prevent undue resistive electrical power losses. Moreover, the DC source of electrical energy used to supply the unipolar motor should have a high energy density.
Sodium-sulfur and other alkali metal batteries and DC sources developed by the assignee of the present invention, Ford Motor Company, are described in the following U.S. Pat. Nos. 3,404,035; 3,404,036; 3,413,150; 3,446,677; 3,468,709; 3,468,719; 3,475,220; 3,475,225; 3,488,271; 3,514,332; 3,535,163; 3,719,531; 3,811,943; 3,951,689; 3,966,492; 3,976,503; 3,980,496; 3,985,575; 3,985,576; and 4,049,889. The drive train of the invention also may be used with heat engines or thermoelectric generators or other low-voltage, high-current DC sources of electrical energy.
The sodium-sulfur batteries and heat engines mentioned above typically produce, for each cell in the case of the sodium-sulfur battery and sodium heat engine, a voltage of about 1.5 volts. Simplicity of construction of the heat engine or sodium-sulfur battery occurs if the various battery cells are connected in parallel rather than in series. If this is done, only a low voltage is produced but a very high current capability is provided. The high energy density capability of this DC source may be utilized to provide a practical energy source for an electric vehicle.
The low voltage produced by a parallel-connected battery as described above may be used to advantage by a unipolar motor. The present invention provides a drive train for an electric vehicle which takes advantage both of the unique properties of the unipolar dynamoelectric machine and the unique characteristics of the DC sources mentioned above.